This holds the prefix length, subnet mask length, that CEF matched for the longest-match search request, indexing into cefPrefixTable alongside the address in cefLMPrefixAddr. This is what actually tells the admin how specific a match CEF found; a search for a /32 host address that resolves with a /8 prefix length here indicates only a very broad, non-specific route covers that destination, which can be an important routing-completeness clue. A search for host address 203.0.113.45 returning a prefix length of 19 here would mean the most specific route CEF has for that address is a /19, not a host-specific /32 or a more precise aggregate.